The term "film", as used herein, means flexible paper like material made from plastics and particularly from polypropylene. Films usually have a thickness not greater than 0.2 millimeters (mm). These materials are used as paper substitutes particularly in the packaging area, for example, inside candy boxes. This material can even be thermoformed into wrappings for individual candy pieces. Films should be distinguished from rigid articles such as sheets, tubes, boards and plates. These films are smoother to the touch than paper equivalents.
Expanded polypropylene offers numerous advantages over non-expanded polypropylene. Objects made of expanded polypropylene exhibit a weight advantage of up to 25% over articles made of non-expanded polypropylene. It is easier to print on expanded polypropylene without having to pretreat the receiving surface as would be required with non-expanded polypropylene.
The art does not provide a method to obtain films, having good physical characteristics, from expanded polypropylene without the addition of significant amounts of polystyrene, polybutadiene and the like. Even with the utilization of highly specialized and specific processes, blowing agents and other additives, the heretofore known conventional processes for forming expanded polypropylene produce cells of uneven structure and give surface irregularities and poor appearance to the resulting film, thereby yielding products of reduced commercial value.
Additionally, previously known methods for producing expanded polypropylene films have certain disadvantages. For example, such films have poor mechanical strength, and more particularly poor tensile strength. Moreover, the specific gravity or density of such films is not always homogeneous. Thus, the films may contain density variations that may reach about 10%.